1. Field of the Invention
This invention relates generally to hydraulic drive systems for self-propelled turf care equipment, and more particularly to a dual-path reversible hydrostatic drive system for a steerable turf care machine having a single variable volume hydraulic pump as a source of hydraulic drive power.
2. Discussion
Commercial turf care machines are typically self-propelled and are often equipped with drive mechanisms for both driving and steering the wheels of the machines. These drive mechanisms typically include a mechanism for transferring the driving torque from a source of power such as an internal combustion engine to at least two of the machine wheels, thereby turning the machine wheels and assisting the machine operator in both propelling and maneuvering the piece of equipment.
While both hydraulic and mechanical drive mechanisms have been commonly used in the past, hydraulic drive mechanisms are being implemented in turf care machines on an ever increasing basis due to decreases in the price of such systems. Specifically, hydrostatic drive mechanisms have long been known to exhibit desirable characteristics such as allowing a machine operator to quickly switch between forward, neutral and reverse modes of operation, and to provide for efficient steering of the machines as well. Dual path hydrostatic drives are used on both zero-turn riding mowers and zero-turn walk-behind mowers for a number of years, as is shown in U.S. Pat. No. 3,616,869 to Rilling and U.S. Pat. No. 4,920,733 to Berrios, which are both hereby incorporated by reference. These drive systems feature a dedicated hydraulic pump and hydraulic motor combination for each of the two driving wheels. Typically, the pumps and motors are located near the driving wheels, and the two pumps are driven by a belt powered by a pulley mounted to the output shaft of the internal combustion engine.
One desirable feature of these dual path machines is their ability to turn and maneuver with considerable agility. This is a direct result of each driving wheel being controlled by the operator independently of the other driving wheel. So, while one wheel is being driven forward at a desired speed, the other wheel can be driven, for example, in the opposite direction at the same speed, thereby allowing the turf care machine to turn on its own axis. Commercial riding and walk-behind mowers having a dual path hydrostatic drive system are available from more than a half-dozen different companies.
A turf care machine operator manipulates a typical dual path hydrostatic drive mechanism through a plurality of levers located on an operator/turf machine interface such as a handle bar structure. These operator control systems typically include both traction controls and speed controls to enhance machine operability and maneuverability. A typical traction control for a dual path hydrostatically-driven machine provides the machine operator with selective control between forward, neutral and reverse modes of operation for each driven wheel, while a typical speed control provides the operator with an overall control for regulating the maximum forward speed at which the machine can be operated. State-of-the art traction and speed controls are disclosed in U.S. Pat. No. 5,343,678 to Stuart and U.S. Pat. No. 5,651,241 to Wegner, which are both hereby incorporated by reference. They are also available on various mid-size commercial mowers including those from Textron Turf Care and Specialty Products of Johnson Creek, Wis., such as the Ransomes(copyright) Bob-Cat(copyright) mid-size mowers Model No. 942215 (a riding mower) and Model No. 930301 (a walk-behind mower), the operations and details of which are respectively described in publicly available Ransomes operator and service manuals.
While the present dual path hydrostatic drive systems that are presently available on commercial turf care machines typically exhibit sufficient performance characteristics, such systems normally require a hydraulic fluid reservoir as well as a pair of reversible variable displacement pumps, thus increasing turf care machine cost. It would therefore be desirable to provide more economic arrangements for the hydraulic drive system which facilitates ease of operation, improves efficiency and/or reduces manufacturing costs.
As such, a first object of the present invention is to provide an improved system for regulating/controlling a hydrostatic drive mechanism of a conventional turf care machine.
A second object of the present invention is to provide an improved system for regulating/controlling the hydrostatic drive system of a turf care machine that allows the operator to adjust the power to the drive system in a way which helps reduce heating of the hydraulic fluid and improves system efficiency.
A third object of the present invention is to provide a system and method for regulating/controlling the hydrostatic drive of a turf care machine that requires only one single pump for driving both drive wheels in a seemingly independent dual path mode of operation, thereby minimizing cost and the number of fluid linkages and parts in such a system.
A fourth object of the present invention is to provide an economically efficient dual-path hydrostatic drive system which has a reduced number of external hydraulic connections for improved system reliability.
To achieve one or more of the foregoing objects, and in accordance with the teachings of the present invention, a single-pump hydraulic drive system for a steerable, self-propelled wheeled vehicle is provided that is designed and adapted for regulating the operation of a self-propelled mower or other piece of turf care equipment or similar kind of the machine. The system of the present invention finds particular utility for midsize commercial mowers and similar kinds of steerable self-propelled wheeled equipment, typically having a net weight of about 140 kg (about 310 pounds) to about 500 kg (about 1100 pounds), which are manually steered and must often be manipulated around obstacles during operation.
A manually-steered self-propelled wheeled vehicle, such as a commercial mower, according to one embodiment of the present invention, is implemented with a single-pump hydraulic drive system having a pair of control valve sets for controlling the volume and direction of fluid from the single hydrostatic pump to a pair of wheel drive motors. The single hydraulic pump, which is preferably a variable volume pump, functions to provide hydraulic fluid power required to drive the wheels. The pump preferably includes a manually-operated maximum volume adjustment mechanism, such as a movable machine speed lever connected to a rotatable trunnion shaft or other volume control input means on the pump, for selectively setting the desired maximum pump displacement at any desired level, from a fraction of the pump""s output, such 20%, all the way up to 100%. Via this machine speed lever, an operator can select the maximum drive speed at which the mower will operate. If desired, this speed control mechanism can be operatively connected in a combined manner on the turf machine so that the speed control lever limits the top speed of the traction controls. The operator traction controls typically include independently-actuated left and right control levers, typically operated by the user""s left and right hands, which are mechanically connected via independent linkages to the conventional control valve operators mounted on the first and second valve sets, which valve sets will shortly be described.
The hydraulic pump is connected to a first valve set, which regulates the volume and direction of fluid to the left hydraulic wheel motor. The left wheel motor mechanically drives the mower""s left driving wheel, and thus controls the direction and speed of rotation of the left driving wheel. Hydraulic fluid may also be partially or completely shunted or bypassed around the left hydraulic wheel motor by the first valve set, in those operating situations where the left driving wheel is not to be driven at all or is not to be driven at a speed as great as the speed of the right driving wheel. The hydraulic fluid, after traveling back from the left wheel motor into the first valve set, or after being shunted through the first valve set, is then transferred through a suitable fluid conduit to a second valve set. The second valve set regulates the direction and flow of the fluid to the lawn mower""s right wheel drive motor. This motor is turn mechanically drives the mower""s right driving wheel, and thus controls the direction and speed of rotation of the right driving wheel. Hydraulic fluid may also be partially or completely shunted around the right hydraulic wheel motor by the second valve set, in those situations where the right driving wheel is not to be driven at all or is not to be driven at a speed as great as the speed of the left driving wheel. After the hydraulic fluid leaves the second valve set, it recirculates through a suitable fluid conduct back to the inlet side of the single hydraulic pump.
The first and second valve sets preferably each contain at least one control valve for regulating the volume and direction of hydraulic fluid discharged therefrom. Preferably each control valve has two outlet ports which are directly piped to the associated wheel drive motor. Each control valve also preferably has a pressure inlet port, a pilot bypass port, and a discharge port. One preferred control valve is an infinitely-adjustable five-port three-position proportional directional control valve for selectively directing fluid to its associated wheel motor. The control valve also preferably has a three-position closed center valve spool arrangement. In the forward and reverse positions, the control valve directs hydraulic fluid to the wheel motor in order to respectively run the wheel motor in the forward or reverse direction. Because the control valve is proportional and infinitely positionable, the flow of hydraulic fluid can be adjusted as desired to achieve any desired rate of speed of the associated wheel, including no speed (that is no rotation), which occurs when the valve spool is in its closed center position.
The left and right operator traction control levers and linkages provided on the mower may take on any conventional or suitable form for providing independent mechanical movements to the control operators of the first and second valve sets. Typically, the control operators of the valve sets will be adjustable between full forward, neutral and full reverse positions. As a first example, on a walk-behind mower employing a single-pump hydrostatic drive system of the present invention, these control levers may be hand grip levers of the type conventionally found on walk-behind mowers having a two-pump dual-path hydraulic drive system. As a second example, on a riding mower where the operator is seated, the control levers may be a pair of hand-operable, pivotally mounted, spring-returned-to-neutral levers positioned along side of, or in between the knees of, the seated operator. As a third example, on a stand-up riding or platform mower, the operator traction control levers may be a pair of hand levers or coupling members that are independently movable and preferably are arranged to pivot about a single axis. The control levers may also be limited in their pivotal movement in at least one direction by a lawn mower handle structure upon which they are mounted. As a fourth example, left and right foot-operated control pedals may be provided. In all of these examples, the control operator of the first valve set receives its command as to the desired direction and volume of the fluid flow into the left wheel drive motor from the left-hand (or left foot) traction control lever through the left-hand linkage. Similarly, the control operator of the second valve set receives its command as to the desired direction and volume of fluid flow into the right wheel drive motor from the right-hand (or right-foot) traction control lever through the right-hand linkage. In this manner, the direction and speed of the rotation of the left and right driving wheels may be accurately controlled. Through manipulation of this pair of controls, the left wheel can go in a forward direction at any desired speed while the right wheel is going in a rearward direction at any desired speed (or vice versa). This functionality enables the operator to turn the mower at an effective zero turning radius, if so desired, just like in traditional dual-path machines.
At least two further variations of the first embodiment of the present invention are contemplated. The control valve of each valve set that determines fluid volume and direction may be a spring-returned to closed-center neutral position valve, meaning the hydraulic flow to the associated wheel motor is zero in the absence of external forces applied to shift the valve away from its centered neutral position. Preferably, suitable spring members are provided internally within the control valve to shift its spool to a closed-center neutral position. Alternatively, the control valve may be spring-returned to one of its full open positions. In either variation, the traction control levers and/or their associated linkages may also be provided with spring members to return the control valve to a neutral position when the mower operator releases the traction control levers. Alternatively, as is common with some commercial walk-behind mowers, the traction control levers and/or their associated linkages may be provided with spring members to return the control valve to a full open position, such as full forward, when the mower operator releases the traction control levers.
A second embodiment or aspect of the single-pump hydrostatic drive system of the present invention for steerable self-propelled wheeled vehicles is provided. This second aspect includes all of the same components found in the first embodiment of the invention. It also includes, within each control valve set, a pressure-operated bypass valve, which is also sometimes called a slave-operated volume compensation valve. These bypass valves are preferably infinitely-adjustable three-port two-position pilot-operated control valves that are spring-returned to an open position in the absence of back pressure in a pilot fluid line typically connected to normal outlet port of the bypass valve. Each bypass valve functions to divert all or a portion of the fluid around its respective control valve within its valve set when a particular wheel is not being driven at all or is not being driven at its near maximum rate of speed so as to match the particular pump output flow. These slave-operated bypass valves include opposed pilot lines connected respectively to the inlet port and the pilot bypass port of the associated control valve. Thus, the nearly instantaneous pressures experienced within the fluid conduits connected to the inlet port and the bypass port serve to control the position of the slave-operated bypass valve. When a sufficient pressure backup is detected in the conduit leading to the inlet port of the control valve, the bypass valve diverts part or all of the fluid flow to the discharge port of its valve set. In this manner, the energy-wasting, heat-generating circulation of the hydraulic fluid through the bypass passages and bypass port of the directional control valve is avoided, and the overall speed of response of the hydraulic drive system as a whole is improved. According to this aspect of the present invention, the bypass valves are preferably mounted directly to or adjacent to their associated control valve. Or both valves can be mounted adjacent to one another on a common pre-ported valve mounting plate or manifold block to achieve a single integrated valve set that minimizes the number of external fluid connections that have to be made to each valve set. In addition, if desired, the single pump and the first and second valve sets may be mounted upon a larger common hydraulic manifold block, base or mounting plate to further minimize the number of external fluid connections between these three sets of hydraulic components within the hydrostatic drive system of the present invention. In all versions of the present invention, the valve sets are preferably positioned in relatively close proximity to the hydraulic wheel motors, which typically directly drive the axles of the driving wheels of the turf care machine.